x=63:7

x=9

Đặt \(A=\left(\dfrac{c}{a}+\dfrac{c}{b}\right)^2\)

\(A=\left[c\left(\dfrac{1}{a}+\dfrac{1}{b}\right)\right]^2\)

\(A=c^2\cdot\left(\dfrac{1}{a}+\dfrac{1}{b}\right)^2\)

\(A\ge\left(a^2+b^2\right)\cdot\dfrac{16}{\left(a+b\right)^2}\)

Cần cm:\(a^2+b^2\ge\dfrac{\left(a+b\right)^2}{2}\)

\(\Leftrightarrow\left(a-b\right)^2\ge0\)(luôn đúng)

\(\Rightarrowđpcm\)

Cách gần tương tự:

\(A=\dfrac{x}{y+1}+\dfrac{y}{x+1}\)

\(A+2=\dfrac{x}{y+1}+1+\dfrac{y}{x+1}+1\)

\(A+2=\dfrac{x+y+1}{y+1}+\dfrac{x+y+1}{x+1}\)

\(A+2=\left(x+y+1\right)\left(\dfrac{1}{y+1}+\dfrac{1}{x+1}\right)\)

\(A+2\ge2\cdot\dfrac{4}{x+y+2}=2\cdot\dfrac{4}{3}=\dfrac{8}{3}\)

\(\Rightarrow A\ge\dfrac{2}{3}\Leftrightarrow x=y=\dfrac{1}{2}\)

p/s:Cần cm bđt \(\dfrac{1}{a}+\dfrac{1}{b}\ge\dfrac{4}{a+b}\)(khá dễ)

\(S=a^2+\dfrac{1}{a^2}\)

\(S=\dfrac{1}{16}a^2+\dfrac{1}{a^2}+\dfrac{15}{16}a^2\)

\(S\ge2\sqrt{\dfrac{1}{16}a^2\cdot\dfrac{1}{a^2}}+\dfrac{15}{16}\cdot2^2\)

\(S\ge2\cdot\dfrac{1}{4}+\dfrac{15}{4}\)

\(S\ge\dfrac{17}{4}\)

Vậy \(MINS=\dfrac{17}{4}\Leftrightarrow a=2\)

đường kính của bánh xe là 0,6 

nha làm ơn làm ơn đi

a) (37 - 17) . (-5) + 23 . (-13 - 17) = 20 . (-5) + 23 . (-30)

= -100 - 690 = -790.

b) Cách 1:

(-57) . (67 - 34) - 67 . (34 - 57)= (-57) . 67 - (-57) . 34 - 67 . 34 + 67 . 57

= 67 . (-57 + 57) - [34 . (-57) + 34 . 67] = 0 - 34 . (-57 + 67) = -34 . 10. = -340.

Cách 2:

(-57) . (67 - 34) - 67 . (34 - 57) = (-57) . 33 - 67 . (-23) = -1881 + 1541 = -340.

số bé 11,25                                                                                                                                                                                                   so lon 45 nha ban

1.6000=1x6000

=>1.6000=6000